Method of machine tool calibration

Abstract

Methods are described for calibrating a turning machine having a first rotatable portion or chuck for holding a workpiece. The first rotatable portion has a first feature associated therewith. The method comprises the steps of using a measurement probe to determine the position of the first feature, rotating the first rotatable portion through an angle, and using said measurement probe to determine the new position of the first feature. The extension of the technique to swivel head mill-turn machines is also described.

1. A method of calibrating a turning machine having a first rotatable portion for holding a workpiece and a second rotatable portion for holding a measurement probe, the first rotatable portion having a first feature associated therewith, the method comprising the steps of;
 (i) using the measurement probe to determine a position of the first feature, 
 (ii) rotating the first rotatable portion through an angle to a new position, 
 (iii) using said measurement probe to determine the new position of the first feature, and 
 (iv) rotating the second rotatable portion through an angle. 
 
     
     
       2. A method according to  claim 1  and further comprising the step of (v) calculating the position of the axis of rotation of the first rotatable portion using the position measurements determined in steps (i) and (iii). 
     
     
       3. A method according to  claim 1  wherein step (ii) comprises rotating the first rotatable portion through an angle of 180°. 
     
     
       4. A method according to  claim 1  wherein step (ii) comprises rotating the first rotatable portion through an angle of less than 180°. 
     
     
       5. A method according to  claim 4  wherein step (ii) comprises rotating the first rotatable portion through an angle of no more than 90°. 
     
     
       6. A method according to  claim 4  comprising an initial step of determining the position of the axis of rotation of the first rotatable portion in a first machine axis. 
     
     
       7. A method according to  claim 6  wherein step (i) comprises orientating the first rotatable portion so that the first feature is located substantially on said first machine axis. 
     
     
       8. A method according to  claim 1  wherein steps (ii) and (iii) are repeated one or more times. 
     
     
       9. A method according to  claim 1  wherein step (ii) comprises rotating the first and second rotatable portions through 90°, and steps (ii) and (iii) are performed four times. 
     
     
       10. A method of calibrating a turning machine having a first rotatable portion for holding a workpiece and a second rotatable portion for holding a measurement probe, the first rotatable portion having a first feature associated therewith, the method comprising the steps of;
 (i) using the measurement probe to determine a position of the first feature, 
 (ii) rotating the first rotatable portion through an angle to a new position, and 
 (iii) using said measurement probe to determine the new position of the first feature, 
 wherein the second rotatable portion is carried by a swivel head, the swivel head being rotatable about an axis. 
 
     
     
       11. A method of calibrating a turning machine having a first rotatable portion for holding a workpiece and a second rotatable portion for holding a measurement probe, the first rotatable portion having a first feature associated therewith, the method comprising the steps of;
 (i) using the measurement probe to determine a position of the first feature, 
 (ii) rotating the first rotatable portion through an angle to a new position, 
 (iii) using said measurement probe to determine the new position of the first feature, and 
 (iv) determining the relative displacement of the axes of rotation of the first rotatable portion and the second rotatable portion. 
 
     
     
       12. A method of determining a position of an axis of rotation of a first rotatable portion as a function of displacement along a translational axis of a turning machine, the turning machine having a first rotatable portion for holding a workpiece, the first rotatable portion having a first feature associated therewith, the method comprising the steps of:
 (A) determining the position of the axis of rotation of the first rotatable portion, the determining comprising the sub-steps of:
 (i) using a measurement probe to determine a position of the first feature, 
 (ii) rotating the first rotatable portion through an angle to a new position, and 
 (iii) using said measurement probe to determine the new position of the first feature, 
 
 (B) translating at least one of the measurement probe and the first rotatable portion along said translational axis, and 
 (C) repeating step (A). 
 
     
     
       13. A method according  claim 12  and further comprising the step of (D) determining the angular alignment of the axis of rotation of the first rotatable portion relative to an axis of the turning machine. 
     
     
       14. A method of aligning a swivel axis turning machine having a first rotatable portion for holding a workpiece and a second rotatable portion for holding a tool or tool accessory, wherein the axis of rotation of the first rotatable portion can be tilted relative to the axis of rotation of the second rotatable portion and the first rotatable portion has a second feature associated therewith, the method comprising the steps of:
 (a) taking a swivel axis turning machine in which the relative displacement of the axes of rotation of the first rotatable portion and the second rotatable portion have been determined using the method according to  claim 11 , 
 (b) using a measurement probe held by the second rotatable portion to determine the position of the second feature, and 
 (c) altering the tilt between the axis of rotation of the first rotatable portion and the axis of rotation of the second rotatable portion and repeating step (b). 
 
     
     
       15. A method according to  claim 14  in which the swivel axis turning machine comprises a swivel head turning machine and the second rotatable portion is carried by the swivel head, wherein step (c) comprises rotating the swivel head to a different orientation and repeating step (b). 
     
     
       16. A method according to  claim 14  comprising the step of determining the position of the pivot point between the axis of rotation of the first rotatable portion and the axis of rotation of the second rotatable portion. 
     
     
       17. A method according to  claim 14  comprising the step of determining tool offset error as a function of the relative tilt between the axes of rotation of the first rotatable portion and the second rotatable portion. 
     
     
       18. A method according to  claim 14  wherein step (b) is repeated with a different displacement between the second feature and the second rotatable portion. 
     
     
       19. A method according to  claim 18  wherein step (b) is repeated using two or more stylus tips, each stylus tip having a different displacement from the second rotatable portion. 
     
     
       20. A method according to  claim 19  wherein step (b) is performed using a multi-tip stylus. 
     
     
       21. A method according to  claim 19  wherein step (b) is performed using two different length styli. 
     
     
       22. A method according to  claim 18  wherein step (b) is performed at least once using the shank of the stylus of the measurement probe to determine the position of the second feature. 
     
     
       23. A method according to  claim 18  comprising the step of determining tool offset error as a function of tool length and the relative tilt between the axes of rotation of the first rotatable portion and the second rotatable portion. 
     
     
       24. A method according to  claim 14  wherein a common feature provides both the first feature and the second feature. 
     
     
       25. A method according to  claim 14  wherein the second feature comprises a datum sphere. 
     
     
       26. A method according to  claim 14  wherein the second feature comprises a shaft comprising two or more datum spheres. 
     
     
       27. A method according to  claim 1  wherein the first rotatable portion of the turning machine comprises said first feature. 
     
     
       28. A method according to  claim 1  wherein the first rotatable portion holds a part comprising said first feature. 
     
     
       29. A method according to  claim 28  wherein the part comprises a protrusion forming said first feature. 
     
     
       30. A method according to  claim 1  comprising the step of using the measurement probe to determine the position of a tool setting device relative to the position of the axis of rotation of the first rotatable portion. 
     
     
       31. Automated turning machine apparatus suitably programmed to implement a method according to  claim 1 . 
     
     
       32. A non-transitory computer readable medium having a computer program for controlling a turning machine, the computer program being such that, when loaded into the computer controller of a suitable turning machine, the machine can implement the method according to  claim 1 . 
     
     
       33. Automated turning machine apparatus suitably programmed to implement a method according to  claim 10 . 
     
     
       34. A non-transitory computer readable medium having a computer program for controlling a turning machine, the computer program being such that, when loaded into the computer controller of a suitable turning machine, the machine can implement the method according to  claim 10 . 
     
     
       35. Automated turning machine apparatus suitably programmed to implement a method according to  claim 11 . 
     
     
       36. A non-transitory computer readable medium having a computer program for controlling a turning machine, the computer program being such that, when loaded into the computer controller of a suitable turning machine, the machine can implement the method according to  claim 11 . 
     
     
       37. Automated turning machine apparatus suitably programmed to implement a method according to  claim 12 . 
     
     
       38. A non-transitory computer readable medium having a computer program for controlling a turning machine, the computer program being such that, when loaded into the computer controller of a suitable turning machine, the machine can implement the method according to  claim 12 .